Mobile units have been very common communication means. Herein mobile units may be portable communication apparatuses, such as mobile phones or wireless modems. For a wireless communication system, the mobile unit transmits information through communicating with a base station. However, the coverage area of a base station is limited. Thus, a number of base stations has to be set up to achieve a coverage area large enough for the mobile unit.
The base station with which the mobile unit communicates is referred to as a serving base station. When the mobile unit is moving out of the coverage area of the serving base station, the mobile unit turns to communicating with another base station as the new serving base station. In this case, the data transfer, storage and synchronization between the mobile unit and the base stations become an issue. As the communication speed and capacity grows, methods proposed under present communication standards would not be good enough.
A method of wireless communication according to the prior art is stated below.
FIG. 1A and FIG. 1B are block diagrams showing a wireless communication network system according to the prior art. This wireless communication network system includes a radio network controller 101, a first base station 103, a second base station 105, and a mobile unit 107. The radio network controller 101 has a network link 11 with the first base station 103 and a network link 13 with the second base station 105. A first link 15 exists between the mobile unit 107 and the first base station 103. The first link 15 is a wireless link. The first base station 103 includes a first register 1031. The second base station 105 includes a second register 1051. The first register 1031 and the second register 1051 respectively have a limited storage capacity for storing data frames transmitted from the radio network controller 101 through the network links 11 and 13.
When storing the data frames transmitted from the radio network controller 101 into the first register 1031, the first base station 103 sends a first response message 111 to the radio network controller 101 through the network links 11. The first response message 111 is for telling the radio network controller 101 that the data frames have been completely received and the first register 1031 still has space for storing data frames. Then the radio network controller 101 could transmit other data frames.
Meanwhile, the first base station 103 transmits received data frames to the mobile unit 107 through the first link 15. When receiving the data frames, the mobile unit 107 sends a second response message 151 to the first base station 103 through the first link 15. The second response message 151 indicates complete receiving of the data frames.
When receiving the second response message 151, the first base station 103 deletes data frames already received by the mobile unit 107 from the first register 1031.
During the above-mentioned process, the first base station 103 has to check if overflow occurs in the first register 1031 before sending the first response message 111. If an overflow is about to occur, the first base station 103 stops sending the first response message 111 to the radio network controller 101. The radio network controller 101 would not be allowed to transmit other data frames until data frames are successfully transmitted to the mobile unit 107 and transmitted data frames are deleted from the first base station 103 to prevent the overflow from occurring. Therefore, no space for storing data frames received by the first base station 103 would not happen in the first register 1031.
A base station could be a serving base station or a neighboring base station in a universal mobile telecommunication system (UMTS), depending on its relation with the mobile unit 107. The first base station 103 is a serving base station and the second base station 105 is a neighboring base station in the foregoing wireless communication system. The mobile unit 107 communicates with the first base station 103 through the first link 15. However, once out of the coverage area of the first base station 103 owing to movement of the user, the mobile unit 107 would switch to communicating with the second base station 105. Accordingly, the radio network controller 101 would simultaneously transmit the same data frames to the first base station 103 and the second base station 105 to prevent any data frames from being lost in case the mobile unit 107 turns to the second base station 105. Therefore, both the first base station 103 and the second base station 105 receive data frames transmitted from the radio network controller 101. And the received data frames are respectively stored in the first register 1031 and the second register 1051.
Nevertheless, the data frames received by the second base station 105 would not be transmitted to the mobile unit 107 and are accumulatively stored in the second register 1051, since the second base station 105 does not have any link with the mobile unit 107 yet. Without an overflow control method, the number of data frames accumulated would larger than the limited storage capacity of the second register 1051. In this case, the data frames in the second base station 105 would be lost or disordered.
Accordingly, a method of controlling data frame overflow in the second register 1051 is needed.
A second link 17 between the mobile unit 107 and the second base station 105 would be set up, as shown in FIG. 1B, when the mobile unit 107 turns to communicating with the second base station 105 instead of the first base station 103. In addition to overflow control, how to synchronize the data frames would be an issue under this condition. Two common methods for frame synchronization in the wireless communication network system are stated below.
FIG. 2 is a schematic diagram illustrating a method for data frame synchronization in the wireless communication network system according to the prior art. According to this method, the mobile unit 107 would send a first exchange message 21 to the first base station 103 when the link quality of the first link 15 is lower than a preset value. The first exchange message 21 includes authentication data of the user and the position of the second base station 105. The first base station 103 would return a first confirmation message 23 after receiving the first exchange message 21. Then the mobile unit 107 would send a second exchange message 25 to the second base station 105. The second base station 105 would return a second confirmation message 27 to the mobile unit 107 after receiving the second exchange message 25. The second exchange message 25 includes a CRue. The CRue is equal to number of data frames already received by the mobile unit 107 through the first link 15. When receiving the second exchange message 25, the second base station 105 would calculate a CS2 equal to number of data frames stored in the second register 1051 and calculate an N by subtracting the CS2 from the CRue. N being larger than zero means there are data frames not yet deleted from the second base station 105 and already received by the mobile unit 107 in the second base station 105. In this case, N of data frames would be deleted from the second base station 105 to achieve data frame synchronization.
However, the transmission speed of the wireless link may be affected by the environment and even be much lower than the network link, so that the information transmission through the wireless link delays. Once the amount of the information to be transmitted is larger than the transmission capacity of the wireless link, the information has to be divided into several pieces and transmitted separately, and then the transmission delay due to the wireless link is even worse.
The second link 17 through which the second exchange message 25 mentioned above is transmitted is a wireless link. However, the second exchange message 25 could not be transmitted through a wireless link at a time if the amount of the CRue included in the second exchange message 25 is larger than the transmission capacity of the wireless link. And the transmission delay gets worse.
Another method for frame synchronization according to the prior art could solve the problem of transmission delay.
FIG. 3 is a schematic diagram illustrating another method for data frame synchronization in the wireless communication network system according to the prior art. The mobile unit 107 would send a first exchange message 21 to the first base station 103 when the link quality of the first link 15 is lower than a preset value, as shown in FIG. 3. However, being different from the method illustrated in FIG. 2, the first base station 103 would send a third exchange message 31 to the radio network controller 101 after receiving the first exchange message 21. The third exchange message 31 includes a CS1. The CS1 is equal to number of data frames already transmitted to the mobile unit 107 by the first base station 103. When getting the CS1 from the third exchange message 31, the radio network controller 101 would send a fourth exchange message 33 including the CS1 to the second base station 105. After receiving the fourth exchange message 33, the second base station 105 would calculate a CS2 equal to number of data frames stored in the second register 1051 and calculate an N by subtracting the CS2 from the CS1. N being larger than zero means there are data frames not yet deleted from the second base station 105 and already received by the mobile unit 107 in the second base station 105. In this case, N of data frames would be deleted from the second base station 105 to achieve data frame synchronization.
After that, to show being ready for communicating with the mobile unit 107, the second base station 105 sends a third confirmation message 35 to the radio network controller 101. The radio network controller 101 would send a fourth confirmation message 37 to the first base station 103. Then the first base station 103 would send a first confirmation message 23 to the mobile unit 107 after receiving the fourth confirmation message 37. The mobile unit 107 would send a second exchange message 25 to the second base station 105 after receiving the first confirmation message 23. The second base station 105 would return a second confirmation message 27 and begin to communicate with the mobile unit 107 after receiving the second exchange message 25.
The second base station 105 uses the CS1 from the first base station 103 instead of the CRue included in the first exchange message 21 to calculate number of data frames not yet deleted from the second base station 105 and already received by the mobile unit 107. The CS1, number of data frames transmitted by the first base station 103, should be equal to the CRue, number of data frames received by the mobile unit 107. Therefore, there is no need to divide certain information into several pieces and transmit separately.
In the above-mentioned method, the first exchange message 21, the first confirmation message 23, the second exchange message 25 and the second confirmation message 27 are sent through the first link 15 and the second link 17, which are wireless links. And the third exchange message 31, the fourth exchange message 33, the third confirmation message 35 and the fourth confirmation message 37 are sent through the network links 11 and 13. Accordingly, the messages have to be sent through the wireless link four times and through the network link four times as well in this method. However, the serving base station would change frequently if the link quality is not good, so that data frame synchronization has to be frequently performed. Therefore, there would be a lot of messages to be sent, though no transmission delay due to the wireless link, in this method.
To conclude, an overflow control method for frame synchronization in the wireless communication network system with less transmission delay and less messages to be sent is required.